Semiconductor lasers in general and VCSELs in particular are widely used as optical sources in fiber-optic communication links. VCSELs may be driven by an RF signal that represents information to be transmitted over the optical fiber. VCSEL performance (slope efficiency and threshold) typically varies as a function of temperature. For example, for a certain value of gain offset, the threshold current of a given VCSEL increases with rising temperature while the output level or intensity of the emitted light decreases with rising temperature.
In operation, however, VCSELs typically must perform over a temperature range on the order of about 80° C. due to variation in ambient temperature and heating in the device package. Therefore, VCSELs commonly require some form of temperature compensation. Conventional temperature compensation approaches fall broadly into two categories, namely optical feedback and active cooling. The optical feedback approach involves deflecting a portion of the light onto a photodetector. The output current of the photodetector is proportional to the intensity of the light incident upon it and is typically fed back as an input to the VCSEL drive circuitry. A significant disadvantage of conventional power monitoring systems is that depending on the beam qualities of the laser, a varying fraction of the radiated light may be incident on the photodetector, which may result in inaccurate monitoring. The major drawbacks of active cooling are 1) it is expensive, 2) thermoelectric (TE) coolers consume relatively large amounts of power and 3) TE coolers are unreliable.
Therefore, conventional systems typically incorporate large area photodiodes to capture a sufficient percentage of reflected light in an attempt to provide accurate feedback over temperature. In addition, the driver circuitry for optical feedback systems is often complex and increases the cost and power requirements of the system.